1. Field of the Invention
This invention relates to a method for preparing a magnetic coating composition which is used to form magnetic layers of magnetic recording media such as magnetic tapes and magnetic disks.
2. Description of the Prior Art
In order to obtain magnetic recording media which exhibit excellent magnetic characteristics, electromagnetic transducing characteristics, or the like, it is necessary to improve the packing density of magnetic grains in the magnetic coating composition which is applied to the substrates of the magnetic recording media. For this purpose, when a magnetic coating composition is prepared, magnetic grains and a solution which contains a binder in an organic solvent should be kneaded at high concentrations and under a high shearing force. In order for this requirement to be satisfied, a technique wherein a two-shaft continuous kneading and mixing machine kneads the magnetic grains and the solution has been disclosed in, for example, Japanese Unexamined Patent Publication No. 62(1987)-41274. Hereinbelow, this type of kneading is referred to as "normal kneading."
In general, magnetic coating compositions are prepared, after the magnetic grains and the solution which contains a binder in an organic solvent have been kneaded, an organic solvent is added to the resulting kneaded mixture in order to dilute it. However, the kneaded mixture obtained from a two-shaft continuous kneading and mixing machine has a high viscosity. If such a mixture having a high viscosity is diluted at a stretch in a flow jet mixture as disclosed in Japanese Unexamined Patent Publication No. 62(1987)-41274, a high shearing force cannot be imparted thereto. Therefore, small lumps remain in the mixture after it has been diluted, and a uniformly diluted mixture cannot be obtained. Also, it takes such a long time to disperse the diluted mixture that even if the diluted mixture is subjected to dispersion processing, it cannot be dispersed to a large extent. When a magnetic coating composition prepared in this manner is applied to substrates, magnetic recording media which exhibit excellent magnetic characteristics, electromagnetic transducing characteristics, or the like, cannot be obtained.
In cases where the kneading and the dilution are sequentially carried out with a two-shaft continuous kneading and mixing machine, instead of the dilution being carried out at a stretch, dilution can be effected gradually. With "dilution kneading," a certain level of shearing force is applied to the kneaded mixture and an organic solvent is added thereto. This process results in the viscosity of the kneaded mixture becoming lower as it is kneaded and diluted. Accordingly, in cases where an ordinary type of two-shaft continuous kneading and mixing machine is directly used to carry out dilution kneading, the problems described below arise. Specifically, a two-shaft continuous kneading and mixing machine comprises a pair of shafts which are spaced a predetermined distance apart from each other and which extend in parallel, a plurality of blade members which are secured to each of the shafts at specific intervals along the axis of each shaft, and a barrel in which the blade members secured to the shafts are accommodated. Shearing force is given to a kneaded mixture in the gaps formed between each pair of blade members, a pair of blade members being made up of blades which are secured to different shafts and which face each other, and in the gaps formed between each blade member and the inner surface of the barrel. If the widths of the gaps are set to values suitable for normal kneading, they will be unsuitable and excessively large for dilution kneading because a kneaded mixture with a decreasing viscosity must be processed. Therefore a high shearing force will not be given to the mixture as it is being kneaded and diluted. Also, the mixture obtained at the time normal kneading is finished has a markedly different viscosity from the organic solvent, which is to be added thereto, and therefore small lumps can easily remain in the diluted mixture. The small lumps, which have the consistency of the undiluted mixture, pass through the aforesaid gaps without being sheared. (This phenomenon will hereinbelow be referred to as the short pass phenomenon.) Therefore, a uniformly diluted mixture cannot be obtained. If the aforesaid gaps are made narrow enough for dilution kneading to be effected properly, a very high load is placed on the blade members during normal kneading, which makes it impossible for normal kneading to be carried out smoothly.